This invention relates to a solid electrolyte for use in a solid electrolytic capacitor and further relates to a solid electrolytic capacitor and a surface-mount transmission line element each using such a solid electrolyte.
In recent years, a solid electrolytic capacitor has been developed which is obtained by forming a dielectric oxide film on a porous body of a valve-acting metal such as tantalum or aluminum by anodic oxidation and then forming on the oxide film an electrically conductive polymer layer as a solid electrolyte. Such solid electrolytic capacitors enable a reduction in equivalent series resistance (hereinafter referred to as “ESR”) as compared with a conventional capacitor using manganese dioxide as a solid electrolyte and thus has started to be used in various applications.
When synthesizing an electrically conductive polymer that forms the conductive polymer layer of the solid electrolytic capacitor, use is made of 3,4-ethylene dioxythiophene (hereinafter referred to as “EDT”), pyrrole, aniline, or the like as a monomer. When forming the conductive polymer as the solid electrolyte of the capacitor, use is mainly made of a chemical oxidation polymerization method that forms the conductive polymer layer by adding an oxidant and dopant to the conductive polymer to cause reactions on the oxide film of the metal porous body (see, e.g.. Japanese Patent (JP-B) No. 3040113, hereinafter referred to as “patent document 1”) or a method that uses, as a base, the conductive polymer layer formed by the chemical oxidation polymerization method and further increases the thickness thereof by the use of an electrolytic polymerization method (see, e.g. Japanese Examined Patent Publication (JP-B) No. H03-61331, hereinafter referred to as “patent document 2”).
On the other hand, a technique is also used that, without carrying out the polymerization on the oxide film of the metal porous body, forms the conductive polymer layer on the oxide film by separately preparing a solution of a soluble conductive polymer, impregnating the polymer solution into the metal porous body, and then drying it into a coating film (see, e.g. Japanese Unexamined Patent Application Publication (JP-A) No. 2001-023437, hereinafter referred to as “patent document 3”). In the case of the technique of patent document 3, the molecular weight of the soluble conductive polymer and the permeability thereof into the inside of the porous body generally have an inverse relationship, while, the electrical resistance of the coating film tends to be proportional to the molecular weight of the conductive polymer. Therefore, if only the soluble conductive polymer solution is used for forming the solid electrolyte of the capacitor, the ESR and capacitance of the capacitor often have a trade-off relationship and, therefore, the example of such use is rare. Practically, soluble conductive polymer solutions are generally used such that (A) a soluble polymer having a large molecular weight is used to prepare a soluble polymer solution that can form a polymer layer with a low resistance although its permeability into a porous body is low, thereby forming a conductive polymer layer with a large thickness near the outermost surface of the porous body in combination with the chemical oxidation polymerization method, and (B) a soluble polymer having a small molecular weight is used to prepare a soluble polymer solution that can facilitate formation of a conductive polymer layer even inside a porous body although its resistance is high, thereby using it as a base in electrolytic polymerization in combination with the electrolytic polymerization method. EDT is a monomer that has started to be widely used and has a large feature that a low-resistance conductive polymer can be obtained and, therefore, also when EDT is used in a soluble conductive polymer, the using method like (A) is often employed. However, at present, even in the using method like (A), the conductive polymer formed by the soluble conductive polymer solution can only form a polymer layer having a resistivity that is several to 100 times higher or more as compared with the conductive polymer formed only by the chemical oxidation polymerization and, further, the stability at high temperatures is also inferior thereto and the resistance increases in a short time, which has been a problem in application to the solid electrolytic capacitor. In the case of a conductive polymer formed by a soluble conductive polymer solution containing EDT as a monomer, the resistance becomes lower as compared with a conductive polymer using another monomer such as, for example, pyrrole or aniline, but, as compared with the conductive polymer formed only the chemical oxidation polymerization, a polymer layer having a resistivity about 2 to 10 times higher can only be formed. Further, it has still been a problem that the stability at high temperatures is inferior thereto.
As examples of soluble conductive polymers containing EDT as a monomer, for example Japanese Unexamined Patent Application Publication (JP-A) No. H01-313521 (hereinafter referred to as “patent document 4”) has proposed and, with respect particularly to the water-soluble conductive polymers, Baytron-P manufactured by H. C. Starck—V TECH Ltd. and so on are on the market. However, those polymers have high electrical resistances and notably increase in electrical resistance at temperatures of about 100° C., and therefore, cannot be applied to solid electrolytes of capacitors as they are.